This invention relates to the preparation and use of mixed matrix membranes containing molecular sieve nanoparticles. These mixed matrix membranes are useful for the separation of fluids and gases, including the separation of carbon dioxide from natural gas.
The use of membranes for separation of both gases and liquids is a growing technological area with potentially high economic reward due to the low energy requirements and the potential for scaling up of modular membrane designs. Advances in membrane technology, with the continuing development of new membrane materials will make this technology even more competitive with traditional, high-energy intensive and costly processes such as distillation. Among the applications for large scale gas separation membrane systems are nitrogen enrichment, oxygen enrichment, hydrogen recovery, removal of hydrogen sulfide and carbon dioxide from natural gas and dehydration of air and natural gas. Also, various hydrocarbon separations are potential applications for the appropriate membrane system. The materials that are used in these applications must have durability, productivity in processing large volumes of gas or liquid and high separation performance in order to be economically successful. Membrane gas separation has evolved rapidly in the past 25 years due to its easy processability for scale-up and low energy requirements. More than 90% of the membrane gas separation applications involve the separation of noncondensable gases: such as carbon dioxide from methane, nitrogen from air, and hydrogen from nitrogen, argon or methane. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications of membrane gas separation have achieved commercial success, including carbon dioxide removal from natural gas and biogas and in enhanced oil recovery.
The membranes most commonly used in commercial gas separation applications are polymeric and nonporous. Gas separation by these membranes is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. This mechanism assumes that each component is sorbed by the membrane at one interface, transported by diffusion across the membrane through the voids between the polymeric chains (or called free volume), and desorbed at the other interface. According to the solution-diffusion model, the membrane performance for a given pair of gases (e.g., CO2/CH4, O2/N2, H2/CH4) is determined by two parameters: permeability coefficient (PA) and the selectivity (αA/B). The PA is the product of the gas flux and the membrane thickness, divided by the pressure difference across the membrane. The αA/B is the ratio of the permeability coefficients of the two gases αA/B=PA/PB) where PA is the permeability of the more permeable gas and PB is the permeability of the less permeable gas. Gases can have high permeability coefficient because of a high solubility coefficient, a high diffusion coefficient, or both. The diffusion coefficient decreases and the solubility coefficient increases with the increase in the molecular size of the gas. For high-performance polymer membranes, both high permeability and selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given amount of gas, thereby decreasing the capital cost of membrane units, and because higher selectivity results in a higher purity product gas with increased efficiency.
The polymers used in membranes provide a range of properties such as low cost, high permeability, good mechanical stability, and ease of processability that are important for gas separation. Polymer modification can improve membrane selectivity. A polymer material with a high glass-transition temperature (Tg), high melting point, and high crystallinity is preferred. Glassy polymers (i.e., polymers below their Tg) have stiffer polymer backbones and therefore let the smaller molecules of gases such as hydrogen and helium pass more quickly, while larger molecules such as hydrocarbon gases permeate the membrane more slowly. However, it is well known that polymers which are more permeable are generally less selective and vice versa. A rather general trade-off has always existed between permeability and selectivity (the so-called polymer upper bound limit). Over the past 30 years, a substantial research effort has been directed to overcoming the limit imposed by the upper bound. Various polymers and techniques have been used, but without much success. For example, many polyimide and polyetherimide glass polymers have much higher intrinsic CO2/CH4 selectivities than that of cellulose acetate. However, these polymers do not have outstanding permeabilities compared to cellulose acetates.
Cellulose acetate (CA) glassy polymer membranes are used extensively in gas separation. Although CA membranes have many advantages, they are limited in terms of selectivity, chemical, thermal, and mechanical stability. One of the immediate challenges facing CA polymer membranes is achieving higher selectivity with equal or greater permeability or to develop other polymer membranes that have the desired combination of properties.
Inorganic membranes, such as purely molecular sieve zeolite membranes, are capable of overcoming the challenges facing polymer membranes. For example, molecular sieve DDR type zeolite membranes have shown much higher CO2 permeability and selectivity for CO2 over CH4 compared to CA polymer membranes. However, these zeolite membranes have poor processability and cannot be fabricated in an economically feasible way with current manufacturing techniques for large-scale applications.
Mixed matrix membranes have been developed that include hybrid blend membranes comprising particles such as molecular sieve particles embedded in a polymer matrix. They combine the low cost and easy processability of the polymer phase with the superior gas separation properties of the molecular sieve phase. These membranes have the potential to achieve higher selectivity with equal or greater permeability compared to existing polymer membranes, while maintaining their advantages. There are many types of mixed matrix membranes reported in the literature such as dispersed solid-polymer mixed matrix membranes which have dispersed solids in polymer phase such as zeolite-polyimide mixed matrix membranes. In contrast to the reports of conventional polymers for membranes in the past 25 years, only a few attempts to increase gas separation membrane performance with mixed matrix membranes of zeolite and rubbery or glassy polymers have been reported.
To date, studies on mixed matrix membranes have used commercially available zeolite particles with relatively large particle sizes in the micron range. See Yong et al., J. MEMBR. SCI., 188:151 (2001); U.S. Pat. No. 5,127,925; U.S. Pat. No. 4,925,562; U.S. Pat. No. 4,925,459; and US 2005/0043167 A1. Commercially available polymer membranes, such as CA and polysulfone membranes, have an asymmetric structure with a thin selective layer. As a consequence, the minimal selective layer thickness of the mixed matrix membranes should be inherently higher than that of most unfilled membranes and the absolute fluxes would be low. Therefore, large zeolite particles having diameters in the micron range are unattractive for mixed matrix membranes. It is highly desirable therefore to use much smaller particles in mixed matrix membranes. One such type of small particles that has been recently developed is the molecular sieve nanoparticles (or so-called nano-molecular sieve particles, e.g. zeolite nanoparticles) which would make it possible to prepare thin, defect-free, nanoparticle filled polymer layers. However, molecular sieve nanoparticles synthesized in the form of a stable colloidal suspension usually contain an organic structure directing agent (SDA) (or so-called template) in the intracrystalline voids. This template has to be removed to form template-free molecular sieve nanoparticles for mixed matrix membrane applications to take advantage of the microporosity of the molecular sieves. The normal direct high temperature calcination that is used to remove the template from such molecular sieves for other uses, such as catalysts, has been found to be unsuitable for colloidal nanocrystals because it leads to significant irreversible aggregation, and non-homogeneous dispersion of the molecular sieves in the polymer. In one commercial example of nanozeolites, the template-free particles are two to three times the diameter of the particles prior to removal of the template. There have been several reports of methods to disperse such nanozeolites in polymers. Most recently, Yan et al. reported a novel technique for the preparation of dispersible template-removed zeolite nanocrystals in various solvents by using an organic polymer network as a temporary barrier during calcinations to prevent zeolite nanocrystal aggregation. See Wang et al., CHEM. COMMUN., 2333 (2000). Smaihi et al. reported the successful preparation of colloidal calcined zeolite nanocrystals by either a grafting-calcination method or a grafting-solvent extraction method. See J. MATER. CHEM., 14:1347 (2004); Gautier et al., NEW J. CHEM., 28:457 (2004). Vankelecom et al. reported the first incorporation of nano-sized zeolites in membranes by dispersing colloidal silicalite-1 in polydimethylsiloxane polymer membrane. See Moermans et al., CHEM. COMMUN., 2467 (2000). Homogeneous polymer-zeolite mixed matrix membranes were also fabricated by the incorporation of dispersible template-removed zeolite A nanocrystals into polysulfone matrix. See Yan et al., J. MATER. CHEM., 12:3640 (2002).
However, it is still desired to develop a process to treat molecular sieve nanoparticles to uniformly disperse them within a polymer matrix. We have now found that surface-functionalized template-free molecular sieve nanoparticles have enhanced dispersity in polar organic solvents as well as an improved ability to adhere to an organic polymer matrix to form a mixed matrix membrane in a void-free fashion and provide for mixed matrix membrane that has an optimized combination of higher permeability and selectivity for gas separation applications.